1. Field of the Invention
This invention relates to a method of controlling the thickness of oriented resin film, and in particular, for preparing biaxially oriented thermoplastic resin films such as a biaxially oriented polypropylene film, a biaxially oriented polyester film, a synthetic paper composed of oriented resin film containing inorganic filler (refer to, for example, Japanese Patent Publication No. 40794/1971), a shrink film and the like. This invention further relates to a method of controlling the thickness of oriented resin film to manufacture a uniform thickness of oriented resin film by controlling nonuniformity in thickness in a transverse direction of the biaxially oriented resin films.
2. Discussion of the Background
Conventionally, a slide lip type T-die is designed so that its lip gap can be adjusted by moving the slide lip along the sliding face of the movable die by using a plurality of gap adjustment die bolts attached transversely to the die (refer to Japanese Patent Publication No. 50179/1988). However, after transverse orientation, the theoretically minimum wavelength on a thickness distribution profile, adjustable by the bolts, is at least two times the product of the mounting pitch between the bolts and the transverse stretching ratio.
Here, the thickness distribution profile is an approximation of a profile of a periodic function (Fourier Series) with a period of 2.pi., having the formula ##EQU1## which is obtained by a harmonic analysis (Fourier transformation) of the thickness distribution of the biaxially oriented film. Further, the wavelength means the product of the film width and the period of the wave (nonuniform thickness) divided by 2.pi..
The slide lip extends transversely integrally; therefore, even when only a particular bolt is adjusted among the plurality of thickness adjusting bolts, not only the lip gap around the adjusted bolt varies but also the lip gaps around the other bolts will be effected by the adjustment. Accordingly, in practice, the adjustable wavelength tends to become longer than that expected. Such a limitation phenomenon commonly occurs regardless of manual or automatic lip gap adjustment. Therefore, such adjustment of the transverse thickness distribution at the T-die naturally has its limitations.
On the other hand, for overcoming this limitation when adjusting the transverse thickness of biaxially oriented resin film at a T-die, a method of controlling the thickness of oriented resin film for reducing thickness nonuniformity is also known (refer to Japanese Patent Laid-open No. 149343/1986), which comprises independently regulating the temperatures of the plural heaters set in a tentering or annealing oven along the oriented resin film in its transverse direction during the film production, thereby controlling the temperature of hot air to be sprayed to the oriented resin film.
Methods of controlling the thickness of biaxially oriented resin film have been investigated, and they comprise combining controlling the thickness by regulating the temperatures of the heater elements and controlling the thickness by regulating lip gap. However, the synergistic effect of both methods produces hunting, which resulting in the thickness distribution of biaxially oriented resin film being manually controlled based on experience as the only method, automatic control based on the combination of thickness controlling methods comprising regulating the temperatures of the heater elements as well as die lip gap never having been conducted.
As a result of the present inventors earnestly investigating these problems, the automatization of thickness control of oriented resin film has been made possible by reading the thickness distribution of the oriented resin film prior to rolling up thereof; separating the readings into modes by means of harmonic analysis (Fourier transformation) using a computer; dividing the modes into two groups of a low frequency component and a high frequency component by a predetermined border mode; and controlling deviation in the profile of the low frequency component by solely regulating to what degree the thickness-adjusting die bolts are tightened as well as controlling deviation in the profile of the high frequency component by solely regulating the temperatures of plural heater elements in an oven.